1. Field of the Invention
The present invention relates to a process for etching a silicon nitride layer. Move specifically, it relates to a process for dry etching a silicon nitride layer on a silicon dioxide layer, useful in a so-called "local oxidation of silicon" (LOCOS) process.
2. Description of the Related Art
A silicon nitride (Si.sub.3 N.sub.4) layer is used as a mask for locally oxidizing a semiconductor bulk, such as a silicon bulk. In this process (LOCOS), the overall surface of a silicon bulk is first oxidized to a thin thickness, e.g., 50 nm. Then, an Si.sub.3 N.sub.4 layer of, e.g., 100 nm thickness is formed on the oxidized surface of the silicon bulk. The Si.sub.3 N.sub.4 layer is patterned with a mask of a patterned resist layer formed on the Si.sub.3 N.sub.4 layer, the resist layer is removed, then the silicon bulk is locally oxidized by using the patterned Si.sub.3 N.sub.4 layer as a mask.
In this local oxidation process, there is the problem of formation of "bird beaks". That is, oxidation of the silicon bulk occurs not only vertically downward from the top surface of a portion of the silicon bulk not covered with the Si.sub.3 N.sub.4 layer as mask but also horizontally toward a portion under the Si.sub.3 N.sub.4 layer. These "bird beaks" create undesired wasteful areas in microcircuit devices, preventing densification of integrated circuit (IC) devices.
It is known that by decreasing the thickness of a silicon dioxide (SiO.sub.2) layer formed on the surface of the silicon bulk and increasing the thickness of an Si.sub.3 N.sub.4 layer or a mask layer for oxidation on the SiO.sub.2 layer, it is possible to reduce the size of "bird beaks" formed during the local oxidation. However, no currently available etching process allows a desired combination of the thicknesses of the SiO.sub.2 and Si.sub.3 N.sub.4 layers. An etching gas comprising fluoromethane (feron), particularly tetrafluoromethane (CF.sub.4), and oxygen (O.sub.2) is typically used for dry etching an Si.sub.3 N.sub.4 layer in the LOCOS process, which gas needs the above mentioned combination of thicknesses of the SiO.sub.2 and Si.sub.3 N.sub.4 layers of e.g., 50 nm and 100 nm, respectively, since the above etching gas has a ratio of etching rate of Si.sub.3 N.sub.4 to SiO.sub.2 of about 2 to 3 at the highest. The above combination of the thicknesses of the two layers, i.e., 50 nm of the SiO.sub.2 layer and 100 nm of the Si.sub.3 N.sub.4 layers, not only requires severe control of the process conditions, but also results in "bird beaks" each having a width or lateral size of 0.3 to 0.4 .mu.m.
If the thickness of the SiO.sub.2 layer is further reduced in the above prior art process, the SiO.sub.2 layer may be over etched during the patterning or etching of an Si.sub.3 N.sub.4 layer on the SiO.sub.2 layer, which causes nonuniform oxidation of the silicon bulk existing under the SiO.sub.2 layer.
There are also known other dry etching gases, for example, a mixed gas of sulfur hexafluoride (SF.sub.6) and chlorine (Cl.sub.2) for etching silicon (Japanese Unexamined Patent Publication (Kokai) No. 58-7829); a gas of SF.sub.6, nitrogen trifluoride (NF.sub.3), fluorine (F.sub.2), or Cl.sub.2 for etching silicon (Japanese Unexamined Patent Publication (Kokai) No. 59-214226); a gas of NF.sub.3 for etching Si.sub.3 N.sub.4 (Japanese Unexamined Patent Publication (Kokai) No. 60-20516); a reactive gas mixture comprising predominantly a fluorocarbon gas doped with carbon dioxide for etching an inorganic insulating layer comprising silicon as a main metal element (Japanese Unexamined Patent Publication (Kokai) No. 58-84113); and a combination of a fluorine-system gas such as SF.sub.6 or CF.sub.4 gas and a carbon-system gas such as ethylene (C.sub.2 H.sub.4) or carbon terachloride (C Cl.sub.4) for etching polycrystalline or crystalline silicon (Japanese Unexamined Patent Publication (Kokai) No. 59-51531). However, some of these publications, disclosing SF.sub.6 as a dry etching gas, do not concern etching of Si.sub.3 N.sub.4 and the others, disclosing a gas for dry etching Si.sub.3 N.sub.4, are not very effective.